The present invention relates to a single step process for the synthesis of nanoparticles of ceramic oxide powders. More particularly, the present invention relates to a single step process for the synthesis of ceramic oxide powders using aqueous and non-aqueous precursors.
Synthesis of nanoparticle ceramic powder is one of the major challenges for the development of advanced ceramics and specialty materials. The most important requirements for the powders of these materials are the availability of uniform nano-sized particles of well defined characteristics without agglomeration and controlled surface properties. Therefore, in recent years the processing of ultrafine, nanoparticles has gained tremendous importance [Chirino A. M. and Sproule R. T., American Ceramic Society Bulletin, Vol.59 (1980), pp.604; Gleiter, Nanostructured Materials, Vol.1 (1992), pp. 1; R. A. Andrievski, Journal of Materials Science, Vol.29 (1994), pp.614]. Nano sized particles, because of their high surface energy and driving force can be densified at much lower temperatures (by several hundreds of degrees) as compared to the large grained powders [W. H. Rhodes, Journal of American Ceramic Society, Vol.64 (1981), pp.19; J. R. Groza and R. J. Downing, Nanostructured Materials, Vol.7 (1981), pp. 749]. The final product will thus preserve the initial grain size of the powder and exhibit unique mechanical, optical, magnetic and electrical properties [M. S. Haji Mahmmod and L. S. Chumbley, Nanostructured Materials, Vol.7 (1996), pp.95; S. W. Mahon, R. F. Cochrane and M. A. Howson, Nanostructured Materials, Vol. 7 (1996), pp.195; C. Suryanarayana, Bulletin of Material Science, Vol.17 (1994), pp.307].
For the synthesis of oxide ceramic materials, chemical methods such as co-precipitation [R. Y. Bratto, American Ceramic Society Bulletin, Vol.48 (1969), pp.739], sol-gel technique [G. I. Missing and M. Kumagai, Journal of American Ceramic Society, Vol. 72(1989), pp.40] spray drying [J. G. Delau, American Ceramic Society Bulletin, Vol.49 (1970), pp.572] and freeze drying [Z. N. Nakagawa et al., Yogyo Kyokuishi 90 (1970) 313] are employed. In the case of co-precipitation methods it is very difficult to obtain homogeneous powders because in multi-component systems, various components precipitate at different pH values. In addition, the precipitate obtained needs calcination at elevated temperatures for getting useful powders. Sol-gel is a multiple step operation which involves calcination for prolonged duration at high temperatures for obtaining powders and processing of high volume off liquids with relatively low yield.
Recently, another processing technique, termed as self-sustained combustion or combustion synthesis, has been used to synthesis fine ceramic powders. This process of combustion synthesis involves rapid decomposition of a saturated aqueous solution, containing metal salts [J. J. Kingsley et al., J. Mater. Sci. 25 (1990) 1305; A. Pathak et al. Nanostructured Materials 8 (1997) 101; S. K. Saha et al. Nanostructured Materials 8 (1997) 29; Raveendranathan and Patil, K. C., J. Am. Ceram. Soc. Bull. 66 (1987) 688]. In the urea-nitrate method proposed by Patil et al the metal nitrate salts and urea are dissolved in minimum quantity of water and the solution was evaporated and burnt. For obtaining the required powder it was necessary to introduce the above solution to a preheated furnace (xcx9c500xc2x0 C.). In the hydrazine complex precursor route [Raveendranathan and Patil, K. C., J. Am. Ceram. Soc. Bull. 66 (1987) 688], the crystalline precursor complex is prepared from a solution containing metal salts and the appropriate hydrazine derivative salt. The cleaned precursor crystals thus obtained are burnt in air to get the required oxide powders. The above procedure is complex and involves less common reagents. The experimental procedure is also reported to be hazardous if not properly handled or controlled [K. C. Patil, Bull. Mater. Sci. 16 (1993) 588]. In another method for the preparation of fine oxide powders Pramanik et al. [S. K. Saha et al. Nanostructured Materials 8 (1997) 29] have used a solution containing metal nitrates, urea (or Tri Ethyl Ammonium Carbonate) and Poly Vinyl Alcohol. In this method, to get phase pure powder of the material, it was essential to calcine the precursor at high temperatures (xcx9c900xc2x0 C.) for prolonged duration. Al these methods of self sustained combustion are limited to preparation of those compounds which have water soluble (aqueous) metal salts. Ceramic compounds having metals whose salts are mostly water insoluble (non-aqueous), like silicon containing compounds, cannot be prepared using the above reported methods.
The main object of the invention is to develop a process for the synthesis of nanoparticle ceramic oxide powders using metal salts that are even insoluble in water.
Another object of the invention is to provide a versatile process for the synthesis of nanoparticle ceramic oxide powders.
Another object of the invention is to synthesise compounds that are single component, two component or even multi-component systems.
Yet another object of the invention is to use a solvent which dissolves all the metal salts required for the preparation of a particular ceramic oxide.
Yet another object of the invention is to provide a process for the syntehsis of nanoparticle ceramic oxide powders without requiring the step of calcination.
It is a further object of the invention to provide a process for the synthesis of nanoparticle ceramic oxide powders without the step of calcination to obtain phase pure ultrafine powder is obtained at relatively low temperatures (xcx9c300xc2x0 C.) using a sand bath/hot plate.
Yet another object of the invention is to obtain any desired single phase ceramic oxide including complex systems in nanoparticulate form.
Another object of the invention is to provide a process for the synthesis of nanoparticle ceramic oxide powders that obviates the disadvantages associated with the prior art processes.
Accordingly, the present invention relates to a single step process for the synthesis of nanoparticles of phase pure ceramic oxides of a single or a multi-component system comprising one or more metal ions, said process comprising,
(a) preparing a solution containing all the required metal ions in stoichiometric ratio by dissolving their respective soluble salts in an organic solvent or in water,
(b) preparing a precursor by complexing the metal ions with a complexing agent while keeping the ratio of the charges of the acid to the charges of the metal ions as unity;
(c) adjusting the nitrate/ammonia content in the system;
(d) heating the system from room temperature to 250-300xc2x0 C.
In one embodiment of the invention, the desired oxide contains (a) one cation selected from the group comprising of Al2O3, ZrO2, TiO2, CeO2, HfO2, MgO, SiO2, (b) two cations of the general formula ABO3, wherein A is Si, Al, Y or Lanthanides, B is Ba, Sr, Ca, Mg or Fe; with general formula AlM2O5, where M=Ti, Zr or Hf; or with general formula Al2NO4, where N=Mg, Ca, Sr, Ba, Zn, (c) three cations with the general formula A(B0.5Bxe2x80x20.5)O6 or A2(BBxe2x80x2)O6, where A is Ba, Sr, Ca or Mg, B is Zr, Hf, Sb or Sn, Bxe2x80x2 is Al, Y or Lanthanides, (d) four cations with general formula (AAxe2x80x2) (BBxe2x80x2)O6, where A and Axe2x80x2 are Ba, Sr, Ca or Mg, B is Zr, Hf, Sb or Sn, Bxe2x80x2 is Al, Y or Lanthanides.
In another embodiment of the invention, the complexing agent is selected from the group comprising of citric acid, EDTA and oxalic acid.
In another embodiment of the invention, the nitrate/ammonia content in the system is adjusted by addition of ammonium nitrate where the precursor is formed in an organic solvent.
In yet another embodiment of the invention, the nitrate/ammonia content in the system is adjusted by the addition of nitric acid and ammonia or ammonium nitrate where the precursor complex is formed in water.
In another embodiment of the invention, water insoluble metal salts are brought into solution by dissolving them in an organic solvent.
In another embodiment of the invention, the metal salts are selected from the group comprising of alkoxides, nitrates, chlorides, sulphates, oxychlorides or any other salts that are soluble in an organic solvent.
In a further embodiment of the invention, the water insoluble oxides and carbonates of the desired metal are dissolved in suitable acids prior to use.
In yet another embodiment of the invention, any metal that can be dissolved to form a solution can be used for making their oxides.
In yet another embodiment of the invention, the organic solvent is selected from the group comprising of alcohols, trichloroethylene, and any other solvent capable of dissolving the complexing agent and any one of the metal salts needed to form the desired oxide.
In a further embodiment of the invention, the organic solvent is selected from the group comprising of ethyl alcohol, methyl alcohol and isopropyl alcohol.
In yet another embodiment of the invention, the combustion is self-ignited and propagated when heated.
In a further embodiment of the invention, the heating is done on a sand bath/hot plate.